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SPATIO-TEMPORAL DYNAMICS OF LARVAL FISH IN A
TROPICAL ESTUARINE MANGROVE: EXAMPLE OF THE
MAHURY RIVER ESTUARY (FRENCH GUIANA)
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Manuscript ID cjfas-2016-0267.R2
Manuscript Type: Article
Date Submitted by the Author: 13-Apr-2017
Complete List of Authors: Rousseau, Yann; Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA), Université de Guyane, CNRS, IFREMER, Blanchard, Fabian; Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA), Université de Guyane, CNRS, IFREMER, Gardel, Antoine; Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA), Université de Guyane, CNRS, IFREMER,
Is the invited manuscript for consideration in a Special
Issue? : 39th Larval Fish Conference
Keyword: Ichthyoplankton, Spatio-temporal variability, Environmental conditions, Estuary, Mangrove
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Title: SPATIO-TEMPORAL DYNAMICS OF LARVAL FISH IN A TROPICAL ESTUARINE MANGROVE:
EXAMPLE OF THE MAHURY RIVER ESTUARY (FRENCH GUIANA)
ROUSSEAU Yann, BLANCHARD Fabian, GARDEL Antoine.
Y. ROUSSEAU. « Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA),
Université de Guyane, CNRS, IFREMER, 97300 Cayenne, France ». Address: CNRS Guyane, Centre de
recherche de Montabo, 275 route de Montabo, 97300 Cayenne, France. (email:
F. Blanchard. « Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA),
Université de Guyane, CNRS, IFREMER, 97300 Cayenne, France ». Address: IFREMER, Domaine de
Suzini, BP477, 97331 Cayenne Cedex, France. (email: [email protected]).
A. Gardel. « Laboratoire Ecologie, évolution, interactions des systèmes amazoniens (LEEISA),
Université de Guyane, CNRS, IFREMER, 97300 Cayenne, France ». Address: CNRS Guyane, Centre de
recherche de Montabo, 275 route de Montabo, 97300 Cayenne, France. (email:
Corresponding author: Yann Rousseau (email: [email protected]). Address : USR Mixte LEEISA,
Université de Guyane, CNRS, IFREMER, Centre de recherche de Montabo, 275 route de Montabo,
97300 Cayenne, France. Phone. +594 594 29 92 83 (GMT-3) / +594 694 22 25 81 (mobile).
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ABSTRACT
The present study focuses on the structure and dynamics of the ichthyoplankton community of the 1
Mahury Estuary (French Guiana) and the factors that influence them. Data were collected on three 2
mangrove sites located in the inner, middle, and outer areas of the estuary. More than 45 000 larvae 3
were collected, representing 31 families and 67 taxa. The community was numerically dominated by 4
few species: Engraulidae was the most abundant family, followed by Gobiidae, Eleotridae, and 5
Sciaenidae. As expected, the most abundant larval taxa were estuarine and mangrove species, with 6
the addition of freshwater species in the inner area of the estuary and taxa with marine affinities in 7
the outer area. The densities of most species were influenced by site more than by season. Temporal 8
variations in the dominant species were influenced largely by their life history strategy, with a 9
majority of the fish species spending their entire life history in mangroves and estuaries. 10
RESUME (French)
La structure et la dynamique spatio-temporelle de l’ichtyoplancton de l’estuaire du Mahury (Guyane 11
française) ont été étudiées ainsi que les paramètres environnementaux pouvant les influencer. Trois 12
sites constitués de mangroves, répartis d’amont en aval, ont été échantillonnés. Plus de 45 000 larves 13
ont été collectées appartenant à 31 familles et 67 taxa. Le peuplement est dominé en nombre par 14
peu d’espèces : les Engraulidae constituent la famille la plus abondante, suivie par les Gobiidae, 15
Eleotridae et Sciaenidae. Comme pressenti, les taxa les plus abondants sont des espèces 16
estuariennes et vivant dans les mangroves, avec des espèces plutôt associées à des conditions d’eau 17
douce dans la partie supérieure de l’estuaire et des espèces avec une affinité pour les eaux marines 18
dans la partie inférieure. Les densités de la plupart des espèces sont plus influencées par le site que 19
par la saison. Les variations temporelles des espèces dominantes sont en lien avec leur cycle 20
biologique, avec une majorité des espèces de poissons ayant un cycle de vie se déroulant 21
entièrement dans les mangroves et estuaires. 22
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INTRODUCTION
Mangroves are known to play an important role in fish nurseries throughout the world (Beck 23
et al. 2001; Dahlgren et al. 2006; Nagelkerken 2009; Sheaves et al. 2013; Abu El-Regal and Ibrahim 24
2014; Lee et al. 2014). In French Guiana, the coast is influenced by the Amazon River, the source of 25
large deposits of nutrients and morphosedimentary dynamics that are unique in the world (Gardel 26
and Gratiot 2005; Proisy et al. 2009; Anthony et al. 2010) and favour the development of mangroves 27
(Artigas et al. 2003; Baltzer et al. 2004; Fromard and Proisy 2010). Mangroves occupy around 80% of 28
the French Guiana coast, covering 700 km². There are two types of mangroves that can be 29
distinguished according to their location; a highly fluctuating seafront mangrove (Walcker et al. 2015) 30
and an estuary mangrove. 31
Although on a worldwide scale mangroves are highly threatened by numerous pressures and 32
are considered endangered by the IUCN (Spalding et al. 2010; Polidoro et al. 2014), the mangroves in 33
French Guiana are relatively well preserved. They are, however, subject to some pressures, mainly in 34
relation to human activity and demographic growth such as urbanisation, hydraulic modifications, 35
agricultural development, pollution and embankment work, although this is still fairly limited. Small-36
scale fishing activities take place in this ecosystem (Blanchard et al. 2011), and two thirds of the 37
species captured by this type of fishing are estuarine or coastal (Cissé and Blanchard 2010). With the 38
high abundance of food and refuge from predation, mangroves are a suitable habitat for many fish 39
species that spend all or part of their life-cycle there (Nagelkerken 2009; Saint-Paul and Schneider 40
2010; Igulu et al. 2014). The role of estuaries as a nursery habitat for fishes has been well established 41
(Beck et al. 2001; McLusky and Elliott 2004; Dantas et al. 2012; Potter et al. 2015). 42
In estuaries, fish species exhibit a diversity of life history (Elliott et al. 2007; Potter et al. 43
2015): Estuarine dependant species spend their entire life cycle in the ecosystem (e.g. resident 44
species) while others spend only a portion of their life in estuaries or make only brief incursions 45
(anadromous, catadromous, marine migrants, marine stragglers; Able 2005). The percentage of these 46
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different categories of estuarine fish varies globally (Whitfield 1999). For example, the estuarine 47
obligate species represent 21% of the total species in South African estuaries (Whitfield 1999) and 48
45% in southern New Jersey, USA (Able 2005). Thus the structure of estuarine larval fish communities 49
also likely mirrors this diversity: Species-specific reproduction occurs inside or outside of the estuary. 50
The connectivity between spawning areas and nurseries fluctuates with species and modes of 51
reproduction. Many species of fish have pelagic eggs and larvae which are carried by tidal currents 52
via passive and/or selective transport to immigrate to and remain in the estuary (Fortier and Leggett 53
1982; Boehlert and Mundy 1988; Sanvicente-Añorve et al. 2011). During the free-living egg and larval 54
stages, environmental conditions within the estuary influence development and survival (Miller and 55
Kendall 2009a). Temperature, photoperiod, tides, latitude, water depth, substrate type, salinity, and 56
exposure are among the most important ecological factors influencing spawning (Miller and Kendall 57
2009a). Temperature, salinity, and food supply are also the most important factors controlling the 58
growth rate of larvae (Miller and Kendall 2009b). 59
Very few studies have been conducted on fish larvae in the mangroves between the Amazon 60
and the Orinoco Delta which are the two largest rivers in South America. Only one study has been 61
carried out in French Guiana (Tito de Morais and Tito de Morais 1994). The geographically closest 62
studies have been undertaken in North Brazil, on the other side of the Amazon and consequently in 63
different environmental conditions (Barletta-Bergan et al. 2002a; b; Bonecker et al. 2007; Sarpedonti 64
et al. 2008; Barletta and Barletta-Bergan 2009; Sarpedonti et al. 2013). 65
Given the limited knowledge of fish larvae in French Guiana and the importance of fishing 66
resources for the growing Guianese population, it is essential to improve our knowledge of the 67
structure and dynamics of larval fish communities in this geographical area. Therefore, the objectives 68
of this study were: (1) to describe the spatio-temporal structure of larval fish communities in an 69
estuarine mangrove over a year, and (2) to identify the environmental factors that are associated 70
with the spatio-temporal dynamics of these communities. 71
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MATERIAL AND METHODS
Study Site
The Mahury River Estuary, situated east of Cayenne, was chosen to study the structure of 72
coastal larval fish communities (Figure 1). The banks of this tidal estuary are colonised by a vast area 73
of mangrove essentially constituted of trees of the genus Rhizophora. The long prop roots of these 74
trees provide protection for numerous fish species for part or all of their life cycle (e.g., Vance et al. 75
1996; Sheaves and Molony 2000; Lugendo et al. 2007b). 76
Situated between 3 and 5°N latitude, French Guiana benefits from an equatorial climate. The 77
temperature is relatively constant throughout the year (mean ~27°C) and annual rainfall of 78
approximately 3000 l m-² per year. Three main seasons are: rainy from January to June, a less rainy 79
(minor rainy) in March-April, and dry from July to December. The mean flows of the Mahury River 80
thus follow this seasonality and range between 10-15 m3s−1 during the low-water period (September 81
to November) and 230-550 m3s−1 during the high-water period (April to June; Lasserre and Collinet 82
2003). The semi-diurnal tide ranges (0.90 - 2.50 m) influence river flow and the sediment re-83
suspension (Orseau 2016), which fluctuates between 80 and >400 mg l-1 (Froidefond et al. 2002). In 84
addition, the Mahury estuary and the whole coast of French Guiana are influenced by the discharge 85
of the Amazon River during the rainy season via the North Brazil current (Muller-Karger et al. 1988). 86
This current transports low salinity waters that are rich in suspended matter (100-150 Mt) and 87
nutrients to the entire coast of French Guiana (Eisma et al. 1991; Gensac 2012). Thus the entire coast 88
of French Guiana is often considered as an open estuarine system. 89
Sampling and sample processing
Every month for 13 months (Feb 2014-2015) we sampled three sites along the estuary, in an 90
upstream gradient (site A, B and C, respectively; Figure 1). Fish larvae were sampled by adapting a 91
method used by Barletta-Bergan et al. (2002b). A plankton net (500 µm mesh, Ø 0.4 m, 2 m long; 92
with a flowmeter) was deployed and towed horizontally under the surface for 10 min in the daytime 93
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during the ebb tide. Each site was sampled in triplicate for 3 consecutive days at the time of the new 94
moon. The choice of the new moon was made following Barletta and Barletta-Bergan (2009) who 95
showed that this period presented the greatest species diversity and the highest densities in a 96
relatively similar, geographically proximate habitat. In total, we collected 117 samples from each site 97
for a total of 351 samples. 98
All samples were fixed in a 4% formalin solution mixed with sea water for 3 days and then 99
preserved in 70% ethanol until they were processed in the laboratory. Identification of specimens to 100
the most precise taxonomic group possible was carried out using determination keys, guides or 101
scientific publications on larvae (Richards 2006; Fahay 2007) or adult fish from the region (Le Bail et 102
al. 1984a; b; Rojas-Beltran 1984; Planquette et al. 1996; Le Bail et al. 2000; Léopold 2004). The 103
number of individuals per taxon was enumerated for each sample and normalised to a fixed volume 104
of filtered water (i.e. 100 m3). Based on the above adult fish literature, we assigned each taxon an 105
ecological guild based upon its preferred habitat (where they spend a majority of their life-cycle): 106
marine, estuarine–marine, estuarine, mangrove, estuarine–freshwater, and freshwater. 107
Concurrent with larval sampling, environmental parameters such as water temperature, 108
conductivity, turbidity, dissolved oxygen, and pH were measured in situ using a multiparameter 109
probe before and after each collection. All environmental variables were measured at the surface 110
and near the bottom. 111
Statistical analyses
As the data were rarely normal, only non-parametric tests were used. Comparison between 112
different factors (sites and seasons) was carried out using PERMANOVA (Permutational multivariate 113
analysis of variance) undertaken on similarity matrices of biotic as well as abiotic data. In order to 114
reduce heteroscedasticity data were first log transformed (log(X+1)). Because each variable 115
measured near the bottom was significantly correlated with the surface variable (Pearson 116
correlation; p<0.0001; r > 0.94) with the exception of turbidity which was not significant at all of the 117
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sites (site A; Pearson correlation; p>0.05), analyses only used the surface measurements of the 118
variables, and with addition of bottom turbidity. 119
The analysis of the general structure of the data was carried out using different exploratory 120
analyses such as PCA (Principal Component Analysis) and CCA (Canonical Correspondence Analysis). 121
PCA on the environmental data enables the identification of correlations between variables and site 122
structure; CCA enables two datasets to be simultaneously analysed by combining the concepts of 123
ordination and regression. For CCA, one dataset contained the community descriptions of the larval 124
fish (dependant variables) while the other contained site-specific of environmental factors 125
(independent variables). For these analyses we used similarity matrices either from the Bray-Curtis 126
index to compare quantitative abundance data (Legendre and Legendre 1998), or from the 127
normalised Euclidean distance to examine the environmental data. To improve the clarity of the 128
visual presentation of the data, environmental data of each site were clustered and delimited by a 129
polygon whose vertices stand for the outermost data for each site. Observations were then 130
confirmed using PERMANOVAs. 131
RESULTS
Larval fish communities
Over the 13 months sampling period, a total of 45 148 fish larvae and post-larvae were 132
collected, belonging to 13 orders, 31 families, and 67 taxa (Table 1). Five families were predominant 133
in the community and represented almost 90% of the total abundance (Table 1). The Engraulidae 134
family alone (8 species) represented approximately one third of the community. Gobiidae (6 species) 135
and Eleotridae (4 species) each comprised 20% of the total amount. Sciaenidae (8 species) 136
represented 10% of the total abundance, and Clupeidae (2 species) made up 5%. The remaining 26 137
families represented around 11% of the total catch. 138
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The majority of larvae surveyed were estuarine/marine species (29 species), mangrove 139
species (10 species), or marine species (9 species) (Table 1). The remaining species were freshwater 140
species (7), estuarine species (3), estuarine/freshwater species (6), or undetermined (3). More than 141
half of the sampled taxa (36) were present at all three sites (Table 1), most of them estuarine/marine 142
and mangrove species, and only one freshwater taxon. The other taxa tended to be marine species. 143
From downstream to upstream, we observed a variation (Figure 2) in the relative abundance of each 144
ecological guild whose spatial distribution showed that marine (M) and estuarine/marine (EM) 145
species were more abundant in the mouth of estuary (site A) than upstream (sites B and C), 146
representing nearly 50% of the assemblage. Then estuarine species increased from 5% downstream 147
(site A) to 27% upstream (site C). Mangrove species were numerous in the three sites but the 148
maximum relative abundance was found in the middle of the estuary (site B) with 60%. 149
Among all sampled taxa, sixteen represented around 90% of the total abundance (Table 1). 150
The principal taxa encountered were the Eleotris sp1 amphidromous species, the anchovy 151
Anchoviella lepidentostole, the goby Ctenogobius stigmaticus and the anchovy Anchoviella 152
guianensis. These 4 species combined comprised more than 50% of the total catch. 153
Spatial and temporal variability of communities
An average of 14±5 taxa (± Standard Deviation) was collected during each sampling effort, 154
but the spatial distribution of these taxa was not uniform. The total number of taxa collected at each 155
site varied from site A with 60 taxa to site B with 48 and C with 46 taxa (Table 1). The average 156
number of taxa varied significantly across sites and season (PERMANOVA, p<0.05; Figure 3 & Table 157
2). The average number of taxa at all sites was significantly lower during the early dry season (12±3) 158
than for the other seasons (>13), which were not significantly different. The average number of taxa 159
at site A (18±3) was significantly higher than at sites B (13±5) and C (10±5), with the latter two sites 160
not significantly different. Species richness varied across sites (Figure 3), with more distinct and 161
significant fluctuations at sites B and C compared to site A (PERMANOVA; p<0.05). Indeed, the 162
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species richness was not influenced by the season at site A contrary to site B where a significant 163
decrease was observed during the early dry season (PERMANOVA; P<0.05), and to site C where a 164
diminution was noted between the wet season and the dry season (PERMANOVA; P<0.05). 165
The overall average abundance of 129±163 individuals per 100 m3 was highly variable (Table 166
2), with abundance at site A (180±169 ind.100m-3) significantly greater than that of site B (102±149 167
ind.100m-3) and site C (104±162 ind.100m-3; the latter two not significantly differing from each 168
other). Although there were month to month fluctuations (e.g. January 2015), there was no 169
significant difference in total larval fish abundance among seasons (PERMANOVA; p>0.05). However, 170
a few significant interactions “season x site” appeared regarding the site level. For site A, the early 171
dry season induced an increase of the total abundance (PERMANOVA; p<0.05), while the trend is 172
opposite for site B and site C (PERMANOVA; p<0.05). 173
Study of environmental conditions
PCA of the environmental data from the three sites over the year enabled the analysis of 174
global variations in environmental conditions (Figure 4). The first two axes explain 73% of the data 175
variability. Axis 1 (45%) was strongly negatively related to temperature, conductivity, and dissolved 176
oxygen, all of which increase during the dry season and decrease with the heavy rainfall during the 177
rainy season. The study sites are thus characterised by strong seasonality: data corresponding to the 178
dry season are on the left of the figure and those for the rainy season are on the right. Between the 179
two main seasons, intermediate values correspond to the minor rainy season and the start of the dry 180
season. Axis 2 explains 28% of the variation and follows a turbidity gradient (near the bottom) and, 181
to a lesser extent, conductivity (Figure 4). Along this axis, sites are ordered according to an 182
upstream/downstream gradient with the most downstream site (site A) on the top left of the figure. 183
Across both PCA axes, data from site A are substantially more variable. 184
PERMANOVA tests confirm these observations of significant differences among sites 185
(PERMANOVA; F=57.0; p<0.001) and seasons (PERMANOVA; F=120.9; p<0.001), and for the 186
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interaction of sites and seasons (PERMANOVA; F=3.6; p<0.001 – Table 2). The principal variables 187
responsible for these differences are temperature, conductivity, and turbidity. Correlation tests on 188
monthly rainfall data (data from Météo-France) and temperature, conductivity, and dissolved oxygen 189
show high seasonality among these three parameters (Figure 5), with average maximal temperature 190
(30.4±0.1°C), conductivity (29290±6737 µS cm-1) and dissolved oxygen values (93.3±2.9%) are 191
attained in October during the dry season (Figure 5) and minimal values (25.1±0.1°C, 106±60 µS cm-1 192
and 66.8±1.3%) in January-February, during the minor rainy season. Turbidity is influenced more by 193
upstream – downstream position than by season (Figure 5). 194
Relationship between environmental factors and community structure
The CCA enabled environmental and species data to be combined in a single ordination 195
(Figure 6). Sites were primarily organised among an upstream/downstream gradient (in accordance 196
with the preceding PCA). Superimposing the species abundance data enabled the identification of 197
the species with the most affinity to some environmental conditions and sites. Thus, for this CCA and 198
despite the low explained data variability of this analysis (23%), species were more closely structured 199
according to site than according to season (Figure 6-A): species with a greater affinity to highly turbid 200
conditions (site A) are at the top right and species with a greater affinity for low turbidity conditions 201
(site C) are at the bottom left. Among the species that were explained by >50% of environmental 202
data (7 species, Figure 6-B), Gobionellus oceanicus, Anchoviella lepidentostole, Colomesus psittacus, 203
and Sciaenidae spp. were abundant at site A, the most downstream site. Conversely, Clupeidae spp. 204
was characteristic of more upstream sites (sites B and C). Anchoviella surinamensis and Anchoviella 205
guianensis are intermediate species that were present at all three sites. 206
DISCUSSION
Environmental conditions of the Mahury estuary fluctuated across seasons and sites. Among 207
the variables measured, temperature, conductivity, dissolved oxygen and, turbidity explained most 208
of the environmental change, similar to other estuarine systems in the world (Blaber 2000; Bianchi 209
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2007; Wolanski and Elliott 2016). The upstream/downstream classification of our sites tracked 210
changes in turbidity and conductivity which was independent of the season. An increasing 211
heterogeneity of data was also noted towards the river mouth, probably due to the marine influence 212
(swell and tide – Day et al. 2012). Otherwise, season (via rainfall) effects on temperature, 213
conductivity, and dissolved oxygen content followed similar patterns to those observed by Barletta et 214
al. (2003; 2005) in North Brazil or in other tropical or subtropical estuaries (Pichler et al. 2015; Blaber 215
and Barletta 2016; Pichler et al. 2017; Possatto et al. 2017). 216
Amongst the world’s mangroves, those of the Indo-Pacific are considered to have the 217
greatest fish diversity with over a hundred taxa (Robertson and Blaber 1992; Thollot 1996; Wei-dong 218
et al. 2003). Although only based on larvae, the 67 taxa that make up our community are a good 219
reflection of diversity encountered in Atlantic mangroves. Moreover, the number of taxa is 220
comparable, if not even slightly higher, to that of similar studies previously carried out in French 221
Guianese mangroves (59; Tito de Morais and Tito de Morais 1994) and in North Brazil (54-63; 222
Barletta-Bergan et al. 2002a; b). Nevertheless, the diversity encountered is well below the 150 fish 223
species surveyed during studies on French Guiana’s adult coastal communities (Puyo 1949; Le Bail et 224
al. 1984a; b; Rojas-Beltran 1984; 1986). In effect, certain species are absent from our samples as 225
their larvae do not have a planktonic phase (e.g., Ariidae which incubate their eggs and young in their 226
mouths until an advanced stage of development; Carpenter 2002). 227
The primary families in the Mahury Estuary (Engraulidae, Gobiidae, Sciaenidae and 228
Eleotridae) are similar to those observed in nearby mangroves but not necessarily in the same order 229
of abundance or in the same proportions. As in numerous other studies (e.g., Tito de Morais and Tito 230
de Morais 1994; Bonecker et al. 2007; Sarpedonti et al. 2008; Bonecker et al. 2009; Costa et al. 2011), 231
Engraulidae dominate our community in contrast to Clupeidae which are minimally represented. 232
With some exceptions, Clupeidae tend to be less abundant at low latitudes (e.g. in Iberian Peninsula: 233
Garrido et al. 2009; Ramos et al. 2017) where they are replaced by Engraulidae (Haedrich 1983; Costa 234
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et al. 2011; Sarpedonti et al. 2013). Gobiidae also represent a large part of our larval fish 235
communities like in most tropical estuaries (Costa et al. 2011; Jiang et al. 2016; Mérigot et al. 2017). 236
A small number of species defines the Mahury larval community and fluctuates along the 237
estuary in relation to conductivity (salinity) and turbidity. As observed in other estuaries in the world 238
(Neira et al. 1992; Tzeng and Wang 1992; Pichler et al. 2017; Ramos et al. 2017), an increase in 239
number of species from upstream to downstream is apparent. This pattern, which is also valid for 240
adult fish community (Jaureguizar et al. 2004; Barletta et al. 2005; 2008), showed that the salinity 241
largely influences the fish assemblages and spatial distribution in estuaries. Seasonal fluctuations of 242
key environmental factors, such as salinity (conductivity), temperature and dissolved oxygen, also 243
highly influenced the species richness in our study. Indeed, although the number of species, closer to 244
the river mouth (site A), was quite stable throughout the year despite the greater heterogeneity of 245
habitat conditions, the season had a greater influence upstream. The lower rainfall and flows during 246
the early dry season led to an increase of salinity and therefore the estuary became inappropriate for 247
freshwater estuarine opportunists (Barletta et al. 2003; 2005), explaining the lowest number of 248
species of this season of transition. Salinity plays a major role in the fish species composition, as 249
many studies have also shown in South America estuaries (Barletta et al. 2005; Blaber and Barletta 250
2016; Pichler et al. 2017). The majority of species found in this study were estuarine/marine and 251
mangrove residents which are associated with highly turbid conditions and complete their entire life 252
cycle within this habitat (Cyrus and Blaber 1987a; b; Neira et al. 1992; Mai et al. 2014). Unlike in 253
subtropical estuaries, ichthyofauna that complete their entire life cycle within tropical estuaries 254
constitute most of total assemblages (Blaber 2000), and thus larvae of these species must be more 255
tolerant to seasonal variations in environment. In addition, the high turbid waters closer to the 256
mouth of the river (site A) probably provide protection through reduced visual cues to predators and 257
are attractive for many larvae of resident or transient species which remain there until they reach a 258
size that reduces their susceptibility to predation (Blaber 2000; Pichler et al. 2015). 259
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The total abundance of Mahury larval fish communities is amongst the highest found in the 260
literature for the North of South America. In effect, with 129 ind. 100m-3, the annual average 261
abundance for all sites is more than 10 times higher than in an estuary of North Brazil (9 ind. 100m-3) 262
where the salinity conditions are similar to those of our estuary (Barletta-Bergan et al. 2002b), and 263
comparable to that obtained in less saline water (oligohaline: 132 ind. 100m-3; Sarpedonti et al. 264
2013). These differences between studies are likely due to different sampling methods, conditions, 265
productivity, habitats, and years. Similarly to species richness, total abundance followed an 266
upstream/downstream gradient with higher abundances at the river mouth where salinity and 267
turbidity were more important. These high abundances in the lower estuary were due to the 268
presence of several dominant species and by a high number of species. Species are mostly 269
estuarine/marine residents and reproduce in the most saline part of the estuary, similar to findings 270
elsewhere (Cyrus and Blaber 1987c; Tito de Morais and Tito de Morais 1994; Whitfield 1994; 271
Barletta-Bergan et al. 2002b; a). Unlike many studies conducted worldwide on adult or larval fish 272
communities (Barletta et al. 2005; Blaber and Barletta 2016; Pichler et al. 2017), no strong seasonal 273
fluctuations of total larval fish abundance were observed at the Mahury estuary scale. Only few 274
variations appeared at the site level between the early dry season and the other seasons. When 275
rainfall decreased during the early dry season, salinity increased in the three sites, triggering an 276
increase of total abundance close to the river mouth (site A) and a decrease upstream (sites B and C). 277
The result obtained at the river mouth is similar to those of many studies in tropical estuaries 278
(Barletta-Bergan et al. 2002b; a; Lugendo et al. 2007a; Huang et al. 2016). The lowest abundance of 279
larvae during the wet season can be linked to the increase of rainfall and freshwater discharge, which 280
was recognized to increase seaward drift of larvae (Barletta-Bergan et al. 2002b). The fact that there 281
was no significant seasonal variation in total larval fish abundance (although there were month to 282
month fluctuations) could derive from highly abundant species which differed according to site. Two 283
species of Gobiidae, Ctenogobius stigmaticus and Gobionellus oceanicus, were responsible for this 284
high abundance in January 2015 at site A (downstream), although Eleotris sp1 and Anchoviella 285
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guianensis were responsible at site C (upstream; Figure 7). Site B, situated between the two sites, 286
presented an intermediate case with the presence of these two species but with less abundance. 287
Consequently there is inter-site variability between species, diversity and abundance. Furthermore, 288
we can notice that the dominant species do not have necessarily a peak of abundance at the same 289
time (Figure 7). Some species seem to reproduce throughout most of the year such as Anchoviella 290
lepidentostole, which presents a peak of larval abundance at the beginning of the dry season 291
(July/August). Conversely, other species reproduce at more specific time of the year. For example, C. 292
stigmaticus and Eleotris sp1 have a short period of reproduction with a maximum abundance of 293
larvae in January/February, while the spawning period of G. oceanicus and A. guianensis lasted from 294
November to February. 295
The larval fish community of the Mahury estuary was numerically dominated by few species, 296
as typically observed for adult fish communities in other estuaries worldwide (Maes et al. 2005; 297
Elliott et al. 2007; Mérigot et al. 2017). Differences in composition and structure of larval fish among 298
the three sites are mainly related to the presence and abundance of dominant species. In the lower 299
estuary, the community was essentially characterised by the presence and/or abundance of 300
Gobionellus oceanicus, Anchoviella lepidentostole, Colomesus psittacus, and Sciaenidae spp. The 301
species G. oceanicus is an amphidromous Goby with adults that are highly abundant in the 302
mangroves and intertidal mudflats of French Guiana (Rojas-Beltran 1986; Le Bail et al. 2012). Farther 303
upstream, Eleotris sp1 is another highly abundant amphidromous species that belongs to the 304
Eleotridae family. Larvae of amphidromous species comprise a significant portion of the Mahury 305
larval fish community, in greater abundance than observed elsewhere (Tito de Morais and Tito de 306
Morais 1994; Barletta-Bergan et al. 2002a; b; Bonecker et al. 2007; Sarpedonti et al. 2008; Bonecker 307
et al. 2009; Costa et al. 2011). Engraulidae larvae are also highly abundant in estuaries in northern 308
South America (Bonecker et al. 2007; Sarpedonti et al. 2008; Bonecker et al. 2009; Costa et al. 2011). 309
As in previous studies (Tito de Morais and Tito de Morais 1994; Bonecker et al. 2007), A. 310
lepidentostole is the dominant Engraulidae species in Mahury Estuary. This coastal species is 311
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common in tropical estuaries (Froese and Pauly 2016) and migrates within the estuaries to spawn 312
(Cervigón 1985). C. psittacus is a euryhaline pufferfish (Tetraodontidae) that is typical of the turbid 313
and brackish coastal and river waters (Cervigón et al. 1993; Léopold 2004). Sciaenidae larvae are 314
often observed in large numbers in the estuaries of northern South America (e.g., Tito de Morais and 315
Tito de Morais 1994; Barletta-Bergan et al. 2002b; Bonecker et al. 2009; Sarpedonti et al. 2013) and 316
other neotropical estuaries (Camargo and Isaac 2005). These species can tolerate large variations in 317
salinity which allow them to live in estuaries that are strongly influenced by freshwater inputs (Saint-318
Paul and Schneider 2010). These results on the spatial variability in the specific composition of larval 319
fish communities in the Mahury estuary should nevertheless be approached with caution, due to the 320
low explained data variability of the CCA (Figure 6). Indeed, the analysis did not highlight the 321
importance of some species that were less abundant (marines and freshwater species) though 322
characteristic of the opposite sites (site A and site C) where environmental conditions were the most 323
contrasted. The species were separated in the estuary along a gradient linked to environmental 324
conditions (mainly salinity and turbidity gradient) according to their ecological guild. Overall, the 325
larval fish assemblages comprised resident species (estuarine/marine and mangrove species). Other 326
species, probably transient species, complete the composition: marine species associated with high 327
salinity downstream (site A), and more estuarine species and freshwater species correlated with 328
lower salinity of upstream (site C). This spatial distribution and structure of the communities have 329
commonly been observed on estuarine larval and adult fish assemblages worldwide (e.g., Barletta-330
Bergan et al. 2002b; Blaber and Barletta 2016; Ramos et al. 2017). 331
In summary, the study showed that the larval fish community in the Mahury estuary was 332
structured according to environmental conditions. Salinity and turbidity were the most important 333
variables influencing the larval fish species composition and abundance. Species richness and 334
abundance gradually changed in the Mahury estuary from upstream to downstream with higher 335
values in the lower estuary where the salinity and turbidity were the most important. Seasonal 336
fluctuations in salinity also determined the larval fish assemblages (to a lesser extent) with the 337
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smallest number of species during the early dry season and the lowest abundance of larvae during 338
the wet season. According to the fluctuation of these environmental variables, the community 339
structure finally highly followed the abundance variations of dominant species which mainly 340
comprised resident species (estuarine/marine and mangrove species). 341
Acknowledgements
This study was carried out as part of the ECOCOT project – Functioning of French Guiana’s coastal 342
ecosystem, financed by European funding (FEDER) and DEAL funds. Thanks to everyone who 343
participated in the numerous fieldwork phases including trainees, CNRS staff and volunteer friends. 344
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patterns of mangrove extinction risk: Implications for ecosystem services and biodiversity loss. In 537
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variation among demersal ichthyofauna in a subtropical estuary bordering World Heritage-listed and 540
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Table 1. Abundance (number of individuals adjusted to a constant volume of 100 m3) of taxa sampled 603
in three sites of the Mahury mangrove estuary and their relative contributions compared to the total 604
catch in each site. 605
Total catch Percent (>0.1%) Ecological
guild Order/Family/Species Ind.100m-3
% Site A Site B Site C
Perciformes 51.3
Eleotridae Eleotris sp1 7 650.5 16.9 1.7 28.8 31.6 MS
Gobiidae Ctenogobius stigmaticus 6 711.6 14.9 24.4 10.5 2.7 MS
Sciaenidae Sciaenidae unid. 3 377.3 7.5 13.0 3.3 2.0 ND
Gobiidae Gobionellus oceanicus 2 249.6 5.0 8.3 3.6 0.7 MS
Eleotridae Eleotridae sp2 1 172.6 2.6 0.2 8.2 1.2 MS
Sciaenidae Micropogonias sp.* 417.9 0.9 1.8 0.2 <0.1 E-M
Sciaenidae Stellifer rastrifer 375.1 0.8 1.7 <0.1 <0.1 M
Sciaenidae Cynoscion acoupa* 323.6 0.7 0.8 1.0 0.3 E-M
Carangidae Oligoplites saliens* 246.3 0.5 0.6 0.7 0.3 M
Eleotridae Eleotridae sp4 134.7 0.3 - <0.1 1.1 MS
Gobiidae Gobiidae unid. 121.3 0.3 0.3 0.4 <0.1 MS
Gobiidae Gobiidae sp2 63.8 0.1 0.1 0.3 <0.1 MS
Eleotridae Eleotridae unid. 46.5 0.1 <0.1 0.1 0.2 MS
Sciaenidae Stellifer microps 41.8 <0.1 <0.1 0.3 <0.1 E-M
Centropomidae Centropomidae sp1* 38.4 <0.1 0.1 <0.1 <0.1 E-M
Carangidae Carangidae ind.* 31.3 <0.1 <0.1 0.1 <0.1 ND
Gobiidae Gobiidae sp3 30.5 <0.1 <0.1 <0.1 <0.1 MS
Centropomidae Centropomidae sp2* 30.4 <0.1 0.1 <0.1 <0.1 E-M
Gobiidae Gobiidae sp1 18.1 <0.1 <0.1 <0.1 - MS
Carangidae Oligoplites sp.* 17.5 <0.1 <0.1 <0.1 <0.1 M
Lutjanidae Lutjanus jocu* 12.9 <0.1 <0.1 <0.1 <0.1 M
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Carangidae Oligoplites sp1* 10.2 <0.1 <0.1 <0.1 <0.1 M
Carangidae Caranx sp.* 8.3 <0.1 <0.1 <0.1 - M
Haemulidae Genyatremus luteus* 7.3 <0.1 <0.1 - - M
Centropomidae Centropomidae unid.* 5.3 <0.1 <0.1 <0.1 <0.1 E-M
Gobiidae Gobioides broussonnetii 5.1 <0.1 <0.1 - - MS
Sciaenidae Cynoscion sp.* 4.5 <0.1 - <0.1 <0.1 E-M
Eleotridae Eleotridae sp3 4.5 <0.1 <0.1 <0.1 - MS
Sciaenidae Plagioscion squamosissimus* 4.1 <0.1 <0.1 <0.1 - E-F
Sciaenidae Cynoscion virescens* 4.1 <0.1 <0.1 - <0.1 E-M
Sciaenidae Sciaenidae sp1 2.3 <0.1 <0.1 - - ND
Carangidae Trachinotus cayennensis 1.2 <0.1 <0.1 - - E-M
Ephippidae Chaetodipterus faber 1.0 <0.1 <0.1 - - M
Centropomidae Centropomidae sp3 1.0 <0.1 <0.1 - - E-M
Carangidae Selene vomer 0.9 <0.1 <0.1 - - M
Clupeiformes 42.8
Engraulidae Anchoviella lepidentostole 6 732.1 14.9 24.8 7.3 5.3 E-M
Engraulidae Anchoviella guianensis 4 302.9 9.5 3.5 10.9 18.5 E
Unidentified Clupeiformes unid. 1 508.8 3.3 0.4 2.9 8.9 ND
Engraulidae Lycengraulis sp. 1 128.7 2.5 3.3 1.8 1.8 ND
Engraulidae Anchovia surinamensis 981.9 2.2 1.7 3.2 2.0 E-F
Clupeidae Clupeidae unid. 971.8 2.2 0.4 3.4 4.0 ND
Clupeidae Harengula sp. 898.6 2.0 0.6 1.7 4.7 E-M
Engraulidae Engraulidae unid. 747.4 1.7 0.8 1.5 3.3 ND
Engraulidae Anchoa spinifer 571.6 1.3 2.3 0.2 0.5 E-M
Pristigasteridae Pellona sp.* 540.0 1.2 0.2 1.1 3.0 E
Clupeidae Sardinella sp. 442.5 1.0 0.1 1.1 2.3 E-M
Engraulidae Lycengraulis batesii 400.4 0.9 0.8 0.8 1.1 E-F
Engraulidae Lycengraulis grossidens 84.0 0.2 0.3 <0.1 0.2 E-M
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Engraulidae Anchovia clupeoides 12.4 <0.1 <0.1 <0.1 <0.1 E
Engraulidae Pterengraulis atherinoides 7.6 <0.1 <0.1 <0.1 <0.1 E-F
Siluriformes 2.8
Auchenipteridae Pseudauchenipterus nodosus 1 085.0 2.4 2.7 2.4 2.0 E-M
Auchenipteridae Auchenipterus nuchalis 56.4 0.13 <0.1 0.3 0.2 E-M
Aspredinidae Aspredo aspredo 41.8 <0.1 0.2 <0.1 <0.1 E-M
Unidentified Siluriformes unid. 39.8 <0.1 <0.1 0.2 0.1 ND
Auchenipteridae Auchenipteridae unid. 10.1 <0.1 <0.1 <0.1 - E-M
Ariidae Sciades herzbergii* 6.1 <0.1 <0.1 - - E-M
Aspredinidae Aspredinichthys tibicen 4.5 <0.1 <0.1 - - E-F
Loricariidae Hypostomus sp.* 3.4 <0.1 <0.1 <0.1 - E-F
Ariidae Cathorops spixii 1.1 <0.1 <0.1 - - E-M
Loricariidae Loricariidae unid.* 1.1 <0.1 - - <0.1 E-F
Tetraodontiformes 2.2
Tetraodontidae Colomesus psittacus 967.0 2.1 2.8 2.1 1.0 E-M
Tetraodontidae Sphoeroides testudineus 21.5 <0.1 <0.1 <0.1 - E-M
Pleuronectiformes 0.3
Achiridae Achiridae unid.* 118.9 0.3 0.3 0.3 0.2 E-M
Achiridae Apionichthys dumerili* 15.9 <0.1 <0.1 <0.1 <0.1 E-M
Unidentified Pleuronectiformes unid.* 5.5 <0.1 <0.1 <0.1 <0.1 ND
Achiridae Achirus lineatus* 5.2 <0.1 <0.1 <0.1 <0.1 E-M
Paralichthyidae Citharichthys spilopterus* 3.8 <0.1 <0.1 <0.1 - E-M
Bothidae Bothidae* 1.1 <0.1 - <0.1 - ND
Paralichthyidae Syacium gunteri* 1.0 <0.1 <0.1 - - E-M
Cynoglossidae Symphurus plagusia* 0.9 <0.1 <0.1 - - M
Achiridae Trinectes sp.* 0.9 <0.1 <0.1 - - ND
Elopiformes 0.3
Elopidae Elops saurus 78.8 0.2 0.2 0.3 <0.1 E-M
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Megalopidae Megalops atlanticus* 57.0 0.1 0.1 0.2 <0.1 M
Anguilliformes <0.1
Ophichthidae Myrophis plumbeus 31.7 <0.1 0.1 <0.1 <0.1 E-M
Unidentified individuals <0.1
Unidentified Unidentified individuals 28.4 <0.1 <0.1 <0.1 <0.1 ND
Unidentified Pseudoleptocephalus unid. 0.9 <0.1 <0.1 - - ND
Mugiliformes <0.1
Mugilidae Mugil curema* 18.0 <0.1 <0.1 <0.1 <0.1 E-M
Gymnotiformes <0.1
Hypopomidae Hypopomidae unid. 17.1 <0.1 <0.1 <0.1 <0.1 F
Characiformes <0.1
Characidae Pristella maxillaris 7.1 <0.1 - <0.1 <0.1 F
Curimatidae Curimata cyprinoides 2.0 <0.1 <0.1 - <0.1 F
Lebiasinidae Nannostomus beckfordi 1.2 <0.1 - - <0.1 F
Characidae Copella carsevennensis 1.0 <0.1 <0.1 - - F
Syngnathiformes <0.1
Syngnathidae Syngnathus pelagicus 3.7 <0.1 - <0.1 <0.1 E-M
Syngnathidae Syngnathidae sp1 1.2 <0.1 - - <0.1 E-M
Cyprinodontiformes <0.1
Poeciliidae Tomeurus gracilis 4.4 <0.1 <0.1 - <0.1 F
Beloniformes <0.1
Belonidae Pseudotylosurus microps 2.4 <0.1 - - <0.1 F
No. of larvae 45 148 17 359 14 299 13 489
No. of species 67 60 48 46
Nota. Data are based on catches between February 2014 and February 2015. Taxa not caught in a site are 606
indicated by . For each taxa, the ecological guild according to its preferred adult habitat is indicated: M, 607
marine; E–M, estuarine–marine; E, estuarine; MS, mangroves; E–F, estuarine–freshwater; F, freshwater. 608
Economic species are marked by *. 609
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Table 2. Sums of squares, mean squares and significant levels for PERMANOVA tests of the 610
environmental data, the number of species and the abundance of fish larvae caught in plankton net 611
in the three study sites between February 2014 and February 2015. 612
Fish data
Environmental data
Source of variation d.f. No. of taxa
Total abundance
All environmental data
SS MS F
SS MS F
d.f.
SS MS F
Main effect
Site
2
1.93 x 108 9.63 x 107 17.64***
3 130.9 1 565.5 12.78***
2
111.69 55.85 56.95***
Season
3
4.30 x 107 1.43 x 107 2.63*
547.33 182.44 1.49
3
355.66 118.55 120.88***
Two-way interaction
Site x Season 6
6.63 x 107 1.10 x 107 2.02*
1 898.1 316.35 2.58*
6
21.00 3.50 3.57***
Residual
105
5.73 x 108 5.46 x 106
12 862 122.5
96
94.15 0.98
Pair-wise tests
Site
A > B & C
A > B & C
A ≠ B ≠ C
Season 3 < 1 & 2 & 4 1 ≠ 2 ≠ 3 ≠ 4
Nota. *P<0·05; **P<0·01; ***P<0·001 613
Multiple comparisons are shown. A, B and C are used for site A, B and C respectively. Numbers are used for 614
seasons. 1, rainy season; 2, minor rainy season; 3, early dry season; 4, dry season. 615
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Figure 1. Location of study sites (A, B and C) sampled monthly between February 2014 and February 616
2015 in the Mahury estuary. Map source: DEAL French Guiana. 617
Figure 2. Relative abundance (%) of each ecological guild of the larval fish assemblages in the three 618
studied sites which were distributed from downstream (site A) to upstream (site C), considering all 619
species collected. Abbreviations used: M, marine; E–M, estuarine–marine; E, estuarine; MS, 620
mangroves; E–F, estuarine–freshwater; F, freshwater. 621
Figure 3. Average abundance (± SD) and number of taxa collected each month on the three study 622
sites during monthly monitoring carried out from February 2014 to February 2015 on the Mahury 623
estuary. Abundance is represented by histogram and the number of taxa by dots linked by a 624
continuous line. 625
Figure 4. Ordination plot resulting from the principal component analysis (PCA) examining the 626
variability of the environmental conditions of the three study sites sampled monthly between 627
February 2014 and February 2015 in the Mahury estuary. To visualise easily the difference between 628
site conditions, enclosing envelopes were drawn with all the transects of each site. Abbreviations 629
used: T°C for temperature; DO for dissolved oxygen. 630
Figure 5. Mean monthly variations (± SD) of (a) temperature, (b) conductivity, (c) dissolved oxygen 631
and (d) turbidity related to the monthly variations in precipitation of samples in the Mahury 632
mangrove estuary during the study period. Precipitations values are averages of daily measurements. 633
Precipitation is represented by histogram and measured variables by points. Pearson correlation 634
between the precipitation and each variable is indicated. Solid lines refer to variables measured at 635
the surface and for plot D the dotted line refers to the variable measured at the bottom. 636
Precipitation: Météo France data. 637
Figure 6. Ordination plot resulting from the canonical correspondence analysis (CCA) carried out on 638
the whole data set collected monthly on the three study sites between February 2014 and February 639
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2015, examining the relationship between environmental conditions and species abundances in the 640
Mahury estuary. All species are represented on figure A and only species explained more than 50% 641
by the environmental variables (7 species) are shown in figure B. To visualise easily the difference 642
between site conditions, enclosing envelopes were drawn with all the transects of each site. 643
Abbreviations used: T°C for temperature; DO for dissolved oxygen. 644
Figure 7. Average abundance (± SD) collected monthly from 5 main species on the 3 study sites from 645
February 2014 to February 2015. The ecological guild of each species according to its preferred adult 646
habitat is indicated in brackets (E–M, estuarine–marine; E, estuarine; MS, mangroves). 647
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0%
20%
40%
60%
80%
100%
Site A Site B Site C
F
E-F
E
MS
E-M
M
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0.0
5.0
10.0
15.0
20.0
25.0
30.0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
700.0
800.0
900.0
F M A M J J A S O N D J F F M A M J J A S O N D J F F M A M J J A S O N D J F
Site A Site B Site C
No. of taxa
Abundance(ind.100m-3)
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Site A
Site B
Site C
Sites
Wet season
Minor rainy season
Early dry season
Dry season
Seasons
Legend :
Dry season
Upstream
Downstream Surface
Turbidity
Bottom
Turbidity
T°C
Conductivity
DO
pH
Axis 1: 45%
Axis
2:
28
%
SEASONS Wet season
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Legend :
Species
Species (all)
Species explained >50%
Site A
Site B
Site C
Sites
Surface
Turbidity
Bottom
Turbidity
T°C
Conductivity DO
pH
Axis 1: 15%
A. lep
A. gui
Scia G. ocea
Clup
A. sur
C. psi
Surface
Turbidity
Bottom
Turbidity
T°C
Conductivity DO
pH
Axis 1: 15%
Axis
2:
8%
Wet season Dry season
(A) (B)
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Downstream
Wet season Dry season
SEASONS
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Draft
1
0
200
400
600
0
20
40
60
80
100
0
200
400
600
0
50
100
150
G. oceanicus (MS)
C. stigmaticus (MS)
A. guianensis (E)
Eleotris sp1 (MS)
Abundance (ind.100m-3
)
0
100
200
300
F M A M J J A S O N D J F F M A M J J A S O N D J F F M A M J J A S O N D J F
Site A Site B Site C
A. lepidentostole (E-M)
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